Much attention has been paid by the paper industry to chemically pre-treating cellulosic suspensions for the purpose of improving the retention and drainage properties thereof. For example, it is common to include various inorganic materials, such as bentonite and alum, and/or cationic organic materials, such as various natural or modified natural or synthetic polymers, in the thin stock for the purpose of improving the papermaking process. These additives are used for pitch control, decoloration of the drainage water or for facilitating release from drying rolls. Starch is often included to improve strength
Process improvements in retention, drainage, drying (or dewatering), and formation (or structure) properties of the final paper sheet are highly coveted. Unfortunately, some of these properties are in conflict with each other. Conventional practice therefore has resulted in the papermaker selecting his additives according to the properties that he judges to be the most important. If, for example, increased filler retention is more important to the papermaker than increased production, then he is more likely to use a cationic polyacrylamide or other very high molecular weight flocculant. If, however, increased production is more important than increased retention, then a coagulant such as aluminium sulfate is more likely to be chosen.
As discussed in U.S. Pat. No. 4,753,710 (Langley et al.), which issued on Jun. 28, 1988, paper stocks may have both an inorganic additive and an organic polymeric material for the purpose of improving retention, drainage, drying and/or formation. For example, a stock may include bentonite, an aluminium sulfate coagulant, and a cationic polymer such as polyethylene imine to improve dewatering. Others have treated paper stock with a filler, a nonionic polyacrylamide, and bentonite. Still others have demonstrated that addition of either a cationic starch or cationic polyacrylamide and bentonite also improves retention. Another process which is believed to result in a suspension having good strength and satisfactory retention includes colloidal silicic acid and cationic starch additives.
In particular, U.S. Pat. No. 4,753,710 provides for the addition of an inorganic material such as bentonite after one of the shear stages, and an organic polymeric material such as a substantially linear, synthetic, cationic polymer (e.g., a cationic polymer flocculant) having a molecular weight above 500,000 and which is added to the suspension before the shear stage in an amount which is at least about 0.03%, based on the dry weight of the suspension. It is also common to include a filler, such as, calcium carbonate, clay, titanium dioxide or talc or a combination, in the cellulosic suspension or paper stock. The filler is preferably incorporated into the stock before addition of the synthetic polymer.
The stock may include other additives such as rosin, alum, neutral sizes or optical brightening agents. It may also include a strengthening agent and this can be a starch, often a cationic starch. The pH of the stock is generally in the range of 4 to 9.
An improvement over U.S. Pat. No. 4,753,710 is disclosed in European Patent Publication No. 0 335 575 (Langley), which was published on Oct. 4, 1989. This patent application was directed primarily to newsprint and board, wherein a low molecular weight cationic polymer, e.g., polyethylene imine, polyamines, polycyandiamide formaldehyde polymers, amphoteric polymers, and polymers of monomers selected from diallyl dimethyl ammonium chloride, diallylaminoalkyl (meth) acrylates and dialkylaminoalkyl (meth) acrylamides, is added to the fiber suspension, followed by addition of a high molecular weight cationic polymer or cationic starch, followed by the addition of bentonite or colloidal silicic acid after the shear stage.
Recently, the papermaking industry has directed its attention to the use of precipitated calcium carbonate and cationic starch as retention aids. It has been discovered that precipitated calcium carbonate-cationic starch systems are useful as efficient binders for improving filler retention, opacity, and strength during papermaking. An example of this is U.S. Pat. No. 4,892,590 (Gill et al.), which issued on Jan. 9, 1990. The Gill patent provides for the addition of 0.13% precipitated calcium carbonate and 1.3% cationic potato starch to a 75:25 hardwood-softwood pulp blend stock containing 20% Albacar 5970 filler pigment which resulted in 89.9% filler retention and 89.0% fiber fines retention. The calcium carbonate component is anionic and colloidal in nature. When used in a papermaking process in the presence of a cationic starch it maximizes filler retention, improves drainage, formation and optical properties while maintaining acceptable strength characteristics in the finished paper.
The present inventor has discovered by extensive experimentation that a chemical treatment program which replaces the high molecular weight cationic flocculant of the cationic coagulant/cationic flocculant/bentonite program disclosed in European Patent No. 0 335 575 with a high molecular weight anionic flocculant results in a substantial improvement of the retention and drainage properties of the treated fine paper stock. This is particularly true when used in conjunction with cationic starch and precipitated calcium carbonate filler at neutral or alkaline pH. At pH values below 6.8, it has been discovered that cellulosic suspensions which include precipitated calcium carbonate filler become unstable, i.e., acid pH will destabilize the carbonate.
The present invention also provides many additional advantages which shall become apparent as described below.